1. Field
The invention relates to devices for the extrusion or mixing of elastomers or plastic products, in particular for the manufacture of tires for wheels.
2. Description of Related Art
A device for the extrusion of elastomers or plastic products conventionally comprises an extrusion screw accommodated in a cavity of a cylinder. The screw exhibits different reliefs, in particular one or a plurality of threads, of which the functions are variable along the length of the screw. The pitch of the thread is thus relatively short in one section of the screw, so that its principal function is to place the rubber under pressure inside the cavity. In another section the thread exhibits a medium pitch, which increases the speed of the rubber. Finally, in another section, the screw exhibits reliefs such as fingers or cutting blades, the principal function of which is to mix the different constituents of the rubber.
Screws of different configurations are used depending on the type of tire to be manufactured and the nature of the rubber. Rather than replacing the screw in its entirety, consideration has been given to its execution in the form of a shaft receiving movable sleeves having reliefs intended to be in contact with the rubber. Depending on the requirements of the manufacturing process, one or other sleeves are arranged on the shaft in order to obtain the desired configuration of the screw, yet without having to replace the screw in its entirety. This arrangement also has the advantage of permitting the replacement of any one of the sleeves when it becomes worn or defective, while retaining the others.
A keyway fitting or even a cylinder-to-cylinder fitting with radial teeth is used as a general rule in order to secure each sleeve rigidly to the shaft of the screw. However, the forces to which the sleeve is subjected when in contact with the rubber are concentrated in the key, which thus presents a high risk of fracture. Furthermore, securing by means of a key has been found to be inappropriate on those sections of screws where the sleeve has a particularly small diameter.